A gigabit passive optical network (GPON) provides high speed data communications, over a fiber optical cable, between an internet service provider (ISP) and end user. A GPON uses point-to-multipoint architecture (1:32) with a fiber optic splitter to serve multiple end-points from a single optical source. For example, the GPON includes an optical line terminal (OLT) at the ISP central office or switching center and a plurality of optical network units (ONU) or optical network terminals (ONT) located near the end users. Each ONU serves an individual end user. The GPON is a shared network, in that the OLT sends a stream of data packets as downstream traffic that is seen by all ONUs. Each ONU reads the content of the data packets that correspond to the particular ONU address. Encryption prevents eavesdropping on downstream traffic. GPON does not need to provision individual fibers between the hub and customer.
The OLT may include a burst mode (BM) transimpedance amplifier (TIA) with an automatic gain control (AGC) in the data receive channel. FIG. 1 shows a conventional TIA 10 within the OLT and including front-end amplifier 12, single-ended to differential (SE2DIFF) amplifier 14, and common mode level (CML) driver 16 in the data receive channel. AGC 20 has an input coupled to the output of SE2DIFF amplifier 14 and an output controlling the gain of front-end amplifier 12. AGC 20 detects the signal level after SE2DIFF amplifier 14 and sets the gain of TIA 10.
Each data packet transfer through the OLT and ONU includes a guard time, followed by a preamble, and then the data payload. A settling time is needed for each data packet after the start of the preamble for TIA 10 to achieve lock or reach steady state operation. AGC 20 typically include circuits with low-pass filtering having a long time constant. As data speeds increase, the time contestant of conventional AGC 20 may exceed the time allocated for TIA 10 to reach steady state during the preamble. A faster AGC is needed for higher data speeds.